The present invention is directed to a system for charging and spraying aerosolized powders for use in powder spray painting and other powder spray applications. In particular, the present invention is directed to a system for generating a flow of charged powder particles to a workpiece with no significant free ion current flowing between the sprayer head and workpiece, while providing efficient, consistent charging of powder particles.
It is well known in the art that the electrostatic charging of powder particles can result in improved coating efficiency in powder painting applications. One method of particle charging, used in a number of prior art devices, is tribocharging. Such devices are generally described in Chapter 3 of J. F. Hughes, Electrostatic Powder Coating (Research Studies Press Ltd. 1984), which is incorporated herein by reference. Tribocharging is a method of charging powder particles through friction with the various surfaces that the particles impact as they travel. For example, powder particles may collide with surfaces within the spray gun itself, such as the spray gun barrel, and may also collide with the walls of the hose connecting the spray gun to the aerosolized powder source. As the powder particles impact these surfaces, frictional forces cause the particle to become charged. Tribocharging results in an opposite electrical charge being deposited on the surface that is contacted.
The tribocharging method has the advantage of requiring no external power supply to charge the powder. Also, since no high-voltage power source is used, no free ions will be created that flow from the spray gun to the workpiece. Free ions flowing to the workpiece damage the smooth surface of powder paint being applied. Also, a free flow of ions can cause a dangerous electrostatic buildup that may eventually result in an electrostatic discharge due to back ionization. Such discharges back from the workpiece to the spray gun may injure the person operating the sprayer, and may otherwise damage the spraying equipment and ruin the powder coating being applied. Considerable research has been conducted in recent years to decrease the resistivity of powders used in powder spray painting applications because of this problem. If a powder with greater conductivity is used, then the free ion charge can bleed to the workpiece and the back ionization problem is reduced. These efforts are unnecessary when tribocharging guns are used.
Tribocharging spray guns also have disadvantages, however. Since there is no electric field between the spray gun and the workpiece in such devices, powder is driven to the workpiece only by the air flow pattern from the sprayer. Thus a significant percentage of the powder will be deposited on the floor of the spray area or on other surfaces other than the workpiece, thus requiring more powder for each application. Also, because of the opposite polarities of powder particles and surfaces inside tribocharging spray guns, powder tends to collect on these surfaces, which reduces the tribocharging effect of the gun as operation continues. This "filming" effect can eventually result in a spray gun that imparts virtually no charge whatsoever to the powder particles being sprayed. This latter disadvantage is believed to be why spray guns that rely exclusively on tribocharging are rarely in use today.
A second method of charging powder particles in a spray gun is to use a corona discharge source in conjunction with the gun. In a typical example, an electrically charged needle is added within the spray gun's barrel. The needle functions as a corona discharge electrode. As the powder passes the tip of the needle, it enters a strong electric field, which causes the powder particles to become charged by ion deposition on the particles. Since this method does not rely on tribocharging effects to charge the powder particles, it is not hampered by filming during prolonged use. However, there are also disadvantages of this method. Since the workpiece in corona discharge systems is functioning as the ground electrode for the corona discharge tip, all of the voltage drop must take place between the discharge tip and the workpiece. The spray gun may be quite far from the workpiece during many spray painting applications. Therefore, a very high voltage is necessary to generate a sufficient electric field within the spray gun barrel to sufficiently charge the powder particles. Also, because of the high voltage drop between the corona discharge source and the workpiece, a large current of free ions flows from the spray gun to the workpiece. This flow of free ions can result in back ionization, which may not only destroy the smooth flow of paint onto the workpiece, but can, as explained above, be hazardous to the spray gun operator and equipment as well.
A third approach is to combine tribocharging and active corona discharge effects into a single gun. A spray gun that utilizes both tribocharging and corona discharge effects will suffer the disadvantages of both; it will gradually lose its ability to fully charge powder particles as a film of powder builds up on the surfaces that the powder contacts, and it will also generate free ion flow that can result in back ionization.
A combined tribocharging/corona discharge system has been developed using a tribocharging gun with a passive corona discharge effect. In this gun, the buildup of charge on the interior surfaces of the spray gun due to tribocharging results in an opposite charge buildup on a corona discharge needle mounted within the spray gun's barrel. Since this charge buildup is due to tribocharging effects, no high-voltage power supply is required. This gun would thus not suffer from the back ionization dangers of conventional corona discharge guns, but would still suffer from the effects of filming that plague traditional tribocharging guns.
U.S. Pat. No. 5,622,313, to Lader et al., discloses a triboelectric powder spray gun that uses a corona discharge needle to discharge the charge buildup on the gun barrel's interior surfaces. The device attempts to solve the problem commonly encountered with tribocharging guns, namely, how to discharge the charge buildup within the gun barrel without discharging the charge on the powder particles being sprayed onto the workpiece. The Lader device attempts to do this by periodically pulsing on and off a corona discharge tip with an opposite polarity to the charge buildup on the interior of the spray gun barrel. This pulsed corona discharge tip eliminates the need for a ground electrode to discharge the barrel's interior contact surfaces. However, since the pulse from the corona discharge tip would also discharge charged powder particles within the spray gun, it could not be activated while charged powder is flowing without adversely affecting the charging of powder particles flowing through the spray gun.
Yet another spray gun design is the low-voltage corona discharge gun. In this device, a conventional corona discharge tip is fitted within the barrel of the gun. Just downstream from the discharge tip is an additional ground electrode in the shape of a ring or washer. This ground electrode replaces the workpiece as the ground in the system. Since the ground in this type of gun is much closer to the discharge tip than in a conventional corona discharge gun, the voltage applied to the corona discharge tip to effect a certain level of charge in the powder particles can be much lower. A reduction in operating voltage from about 80 kV in a traditional corona discharge gun down to about 6 kV for a low-voltage gun is typical. Thus a smaller, lower-cost voltage supply can be used for these guns. Also, since the space within which the powder is charged is confined to a small area within the barrel of the spray gun itself, the efficiency of the charging process is greatly increased.
Although Hughes claims that there is no free ion flow from a low-voltage spray gun to the workpiece, experiments have shown that this statement is not accurate. While perhaps some of the free ion flow strikes the ground electrode and thus does not travel beyond the spray gun, a significant free ion flow current still flows from the low-voltage gun to the workpiece. This device therefore still suffers from the back ionization problems of conventional corona discharge spray guns, although the severity of this problem may be somewhat reduced.
Another disadvantage of the low-voltage gun is the voltage drop between the spray gun and the workpiece. A voltage drop between the spray gun and workpiece is desired because the electric field thus created will guide charged particles to the workpiece, increasing the efficiency of the coating operation. There is some voltage drop between the low-voltage gun and workpiece due to charge that builds up within the insulated spray gun barrel (and the diffuser, if present, at the end of the barrel) downstream from the ground electrode. Because this charge builds up all along the barrel walls and the diffuser, the resulting electric field pattern is scattered, and does not efficiently direct charged particles to the workpiece. Also, since this electric field depends upon charge build-up within the spray gun, it is variable over time, and will not exist at all when the gun is first used. Thus the electric field between the spray gun and workpiece is not as steady and reliable as that produced by conventional high-voltage corona discharge guns. The overall result is that the powder spray process in the low-voltage gun is far less efficient than for the conventional high-voltage corona discharge guns, where the direct voltage drop between the gun and the workpiece serves to precisely guide the charged particles to their target.
Still another disadvantage of the low-voltage gun is that it does not efficiently charge particles that pass through the spray gun barrel. Due to the electric field pattern between the corona discharge tip and the ground electrode, the volume through which corona ion current flows is quite thin, running only from the tip to the upstream edge of the ground electrode ring. The field lines for this electric field run essentially perpendicular to the flow of powder through the spray gun, and thus the powder has only a brief opportunity to be charged. By contrast, a conventional high-voltage gun has electric field lines that run from the corona discharge tip to the workpiece, essentially parallel to the powder flow direction, such that the powder remains in the field in the presence of corona ions for a considerable time and is more efficiently charged.
As explained above, each of the prior art powder spray gun systems has certain disadvantages. It would be desirable to develop a spray gun that produced an electric field with a corresponding corona ion concentration that efficiently charged the powder particles passing through the spray gun's barrel; produced a smooth electric field between the spray gun and the workpiece to direct charged particles to the workpiece; maintained a constant charging efficiency over time without required periods of disuse for discharging; but did not produce a significant free ion flow between the spray gun and the workpiece.